1. Field of the Invention
The field of the invention is that of the coding of color images. More specifically, the invention relates to the coding of color images constituted by a set of at least two independent, monochromatic channels or images. This coding may notably be the coding of three signals, red, green and blue, coming from pick-up instruments or cameras and used for the reproduction of the images, for example on a television screen.
The coding of these signals is very often necessary, to compress the information that they contain. This is notably the case when it is desired to transmit them, in order to reduce their bit rate, or store them, for example on tape or digital disk, in order to reduce the space that they occupy.
The invention can be applied as much to the coding of still images as to that of motion images. It depends neither on the standard of the image nor on the type of prior and/or subsequent processing undergone by the signals.
2. Description of the Prior Art
There already exists an international standard for the coding of still color images, the ISO 10 918 standard, also known by the designations JPEG Joint Photographic Experts Group) and ISO-DCT. This standard is unaffected by the type of color space used to represent a color picture. Indeed, it considers a color image to be a plurality of monochromatic images processed independently.
The JPEG standard enables the simultaneous processing of up to 256 monochromatic channels. In practice, there are 1, 3 or 4 channels. In certain cases, however, there is a larger number of images. Thus many satellites, for example meteorological observation satellites, deliver the three standard color signals as well as several signals corresponding to shots taken in the infrared range. It can be clearly seen that, in this case, it is very important to code these images, in order to reduce their transmission bit rate, limit the space needed for their storage or, again, enable their delivery by the telephone system.
There is a similar known standard for the coding of motion color images. This is the ISO 11172 standard or MPEG (Motion Picture Experts Group) standard.
In both cases, the color image is therefore considered to be the sum of three (or more) independent channels or matrices of pixels. The choice of these channels is therefore a matter of application.
Three color space representations are presently used as a matter of common practice: YUV, RGB (red, green, blue) and JMC (yellow, magenta, cyan) or JMCK.
The YUV representation is derived from the CCIR recommendation 601. The three dot matrices are respectively the matrices of the luminances with full resolution (Y) and two color difference matrices D.sub.R (U) and D.sub.B (V) filtered with half-resolution in both directions, horizontal and vertical. This choice results from the implicit assumption, which is an obvious one in the context of a television studio, that the conversion between the image-taking color space and the coding space is done on analog signals, for which the matrix processing and filtering operations present no technological difficulties.
The analog-digital conversion done after these operations leads to a representation of 16 bits per pixel instead of 24 (a monochromatic pixel being coded on an octet). An ADCT (adaptive discrete cosine transform) compression is done on this already compressed image by analog means. This transformation assigns quantization values variable as a function of the rank or order of the coefficient considered and of the quality or bit rate desired.
At restitution, it is necessary to carry out a reverse matrix processing operation which is generally obtained after a digital-analog conversion. However, in a microcomputer or computer communications environment, the matrix processing and filtering should be done digitally, for want of specialized analog circuits in the equipment of this range. These operations prove to be particularly costly in terms of processing time.
Moreover, in addition to the technical difficulties, the operations of sub-sampling filtering in both directions introduce fuzziness during the reconstruction of the images.
The RGB color space representation is the direct representation of the image-shooting step and of the reproduction step. It calls for no filtering operation but gives the compressor a representation with 24 bits per pixel (or three independent representations with 8 bits per pixel). Its performance characteristics in terms of compression/quality ratio are therefore lower than those of the YUV representation, which has only 16 bits per pixel before compression. By contrast, it is easier to implement in a microcomputer or computer communications context, owing notably to the fact that there is no filtering.
The trichromatic or quadrichromatic JMC(K) or YMC(K) representation is the one resulting from a separation of colors for printing. It is well suited to the printed reproduction of documents but, for the purpose of screen display, it requires a return to the RGB space. Furthermore it is constituted by a primary representation, before compression, with 24 to 32 bits per pixel. It is well suited to the world of publishing and of computer-aided page make-up but less well suited to computer communications.
It is thus seen that the RGB representation, which is more universal, is the most promising one from the viewpoint of the applications and of the simplicity of the processing operations. However, it proves to be less efficient as regards the data compression algorithms.
The YUV representation is often preferred to the RGB representation, despite the complexity of the processing. The main drawback of the YUV representation is the need for a filtering operation which, when done by digital methods, calls for a prohibitive amount of time if it is done by software, or else calls for the use of a DSP (digital signal processor) or specialized circuits. Moreover, this filtering naturally leads to a loss of image quality, notably by producing fuzzy contours.